1. Field of the Invention
The present invention relates to an apparatus and method for determining the photosynthetic activity of a plant by determining the chlorophyll fluorescence of the plant.
2. Description of the Prior Art
It is important in many areas of plant husbandry to determine the physiological condition of a plant or group of plants. For example, the forestry industry replants millions of seedlings every year. These seedlings are grown in a controlled environment and are preferably transplanted in the field during certain very specific and critical periods during seedling development. It is difficult to determine, by physical appearance alone, when a seedling has developed to a stage when transplant can occur with minimal interference with the growth cycle of the seedling. If a seedling is transplanted at the wrong time the possibility of impairing the growth of the seedling is increased. In some situations the trauma to the seedling may be such, or the development of the seedling may be at a particularly vulnerable time, that death of the seedling occurs. In addition, it is desirable in various situations to cull out plants which have been damaged by frost, high light intensity, herbicides or other inhibitors to ensure optimal plant viability and efficiency of plant husbandry operations.
In addition, it can be difficult to determine from external plant appearance whether or not the light intensity in a greenhouse or nursery setting may be optimal for plant health. Similarly a determination of plant stress, effects of fertilizer and water regimes and effects of physical damage on the plant's health is difficult if not impossible to determine based on the external appearance of the plant.
It is well known that fluorescence emission from plants and plant material is an accurate indication of the photosynthetic activity of the plant and consequently the general health and development of that plant. Devices which measure plant fluorescence in order to determine the general condition of a plant are also known. These apparatuses generally utilize an artificial light source to induce photosynthesis in the plant or portion of the plant thereby inducing fluorescence in the plant. This fluorescence can be detected by a photodetector set at the specific waveband of light corresponding to these fluorescence emissions. Alternatively, apparatuses exist wherein the light source is maintained at a level which does not induce photosynthesis and the effect of modulated high light intensity on the signal from the weak measuring light is monitored. Such a device is described in European Patent Application Number 86304543.1 published Jan. 21, 1987 under number 0209247. This device measures the CO.sub.2 uptake of the plant and the light absorbed by the plant.
FIG. 1 is an example of several fluorescence emission curves from a white spruce seedling measured at various times. The CO.sub.2 uptake rate taken at each sampling interval is also indicated on the graph for comparison purposes. The term APS is an abbreviation for "apparent photosynthesis rate" of the seedling. Note that the relative fluorescence emission is an indication of the "hardening off" of the seedling during the late fall or early winter season. The determination of the occurrence of "hardening off" in a seedling is important in indicating when a seedling may be safely lifted and transferred to winter storage.
If meaningful analysis and recommendations are to be provided to the greenhouse operator it is important that reproducible measurements concerning plant fluorescence be obtained and that the measurements be provided to the operator in an understandable manner. Prior art apparatuses and methods do not provide an accurate, convenient and reproducible measurement of plant fluorescence and therefore comparison between plants or between the same plant at different times does not provide the most reliable data for interpretation. Furthermore, without accurate, easily acquired, reproducible data, comparison of sample fluorescence curves with previously acquired data bank fluorescence curves obtained under established conditions, is difficult. Specifically, fluorescence curve reproduceability is affected by several factors which are not adequately monitored in prior art devices, including:
(a) differences in excitation light intensity on the plant; PA1 (b) automatic compensation for system dark signals, that is signals caused by the detection circuitry in the absence of a fluorescence signal; PA1 (c) automatic compensation for straylight signals caused by background light and fluorescence in the sphere when no plant is present; PA1 (d) sufficiently reliable automatic determination of fluorescence emitted from the plant before the onset of photochemistry (the Fo level); PA1 (e) the application of light intensity in the integrating sphere on the plant which is insufficient to induce acceptable rates of photosynthetic activity in the plant; and PA1 (f) automatic determination of net light absorbed by the sample in the sphere as a means to evaluate sample size. PA1 F.sub.VAR (t) is the corrected fluorescence value at time t, PA1 F.sub.meas (t) is the measured fluorescence at time t, PA1 L.sub.st is the straylight signal, and PA1 Ds is the dark signal PA1 F.sub.VAR is the normalized and corrected fluorescence value, and PA1 Fo is the initial fluorescence
There is a need for an apparatus and method for determining plant fluorescence in a reproducible manner and which can be accurately compared with fluorescence of other plants, or with the same plant over several periods of time. As well, there is a need for an apparatus and method for determining plant fluorescence which can be accurately compared with appropriate data bank fluorescence curves of a plant whose fluorescence was measured under more established conditions in order to provide accurate analysis of the health or development of the plant sample and to provide recommendations concerning the care or transplant of that plant.